1. Field of the Invention
The present invention relates to a semiconductor laser device, and more particularly, to a semiconductor laser device used for optical communication, having a wavelength of 1250 nm or more and provided with a dielectric film on a resonator facet of a semiconductor laser.
2. Description of the Related Art
As the amount of communication demand drastically increases, efforts are being made to realize large capacity communication systems. Transmission systems in optical communication mainly use 1.3 μm band signal light and 1.55 μm band signal light.
The 1.55 μm band signal light involves only small optical fiber loss and is used as signal light in a long-distance communication system. This is called “interurban communication (trunk system)” and used for communication between megalopolises, for example, between Tokyo and Osaka.
On the other hand, the 1.3 μm band signal light involves greater optical fiber loss but has less wavelength dispersion and is used as signal light in a short-distance communication system. This is called, for example, “intra-urban communication” and used for communication within a megalopolis. Furthermore, the 1.3 μm band signal light is also used for communication between a base station and individual households called an “access system.”
Long-wavelength semiconductor lasers that generate signal light having a wavelength of 1.25 μm or more including such signal light are also required to perform high-speed response at a low operating current.
Facet emitting semiconductor lasers are normally used as long-wavelength semiconductor lasers for optical communication. An facet emitting semiconductor laser generally has a pair of mutually opposed facets formed in a crystal by cleaving or etching, allows light to travel between the facets through reflection and thereby obtains light feedback necessary for laser oscillation. Such a semiconductor laser is known as a Fabry-Perot semiconductor laser.
In addition, lasers such as a distributed feedback semiconductor laser using a diffraction grating and distributed reflector semiconductor laser are known, and many such lasers have a structure using reflection on an facet in addition to light feedback using a diffraction grating.
For example, the semiconductor laser body of a Fabry-Perot semiconductor laser has a resonator having a multilayered structure of an n-type clad layer, active layer and p-type clad layer. A single-layer film made of Al2O3 and having a thickness of λ/2 is formed on the front facet of this resonator.
Here, λ is an in-medium wavelength and defined as λ=(wavelength in vacuum of light that emits from the semiconductor laser body)÷(refractive index of medium through which laser light propagates).
When a single-layer film made of Al2O3 and having a thickness of λ/2 is formed on the facet of the resonator, the reflectance of light on the facet is determined by a refractive index of the semiconductor making up the resonator with respect to air. For example, when the refractive index of the semiconductor is 3.2, the reflectance is on the order of 30%.
Furthermore, a multilayer film made up of, for example, an SiO2 film and Si film is formed on a rear facet of the resonator. In the case of a multilayer film composed of three layers of an SiO2 film, Si film and SiO2 film, each layer having a thickness of λ/4, the reflectance becomes approximately 60%. When two layers of a Si film and SiO2 film are additionally stacked outside the SiO2 film, the reflectance becomes approximately 90%.
Furthermore, in the case of a distributed feedback semiconductor laser, for example, a diffraction grating is provided along an active layer of the resonator and an antireflection film is formed on the front facet and a high reflection film similar to a Fabry-Perot semiconductor laser is formed on the rear facet.
Laser oscillation is generally started when a current equal to or higher than a predetermined value is passed through a semiconductor laser. The current value in this case is called a “threshold current.” The threshold current is a current that does not contribute to light emission of the laser and is generally preferred to be low. The threshold current corresponds to a current value at which a gain produced by current injection is balanced with resonator loss. Here, the resonator loss is the sum of internal loss (absorption loss or the like) and mirror loss.
The mirror loss is greater as the reflectance of the facet is lower, and therefore increasing the reflectance of the facet causes the mirror loss to decrease and can reduce the threshold current.
Furthermore, in the case of a distributed feedback semiconductor laser or distributed reflector semiconductor laser, the degree of influence of a diffraction grating on light (normalized coupling coefficient) also has a large influence on mirror loss and the facet reflectance also has a large influence. However, as for the normalized coupling coefficient, increasing the normalized coupling coefficient allows mirror loss to be reduced.
However, increasing the reflectance of the facet of any semiconductor laser causes light density on the facet to increase.
For example, with regard to a Fabry-Perot semiconductor laser, the light density on the front facet of a Fabry-Perot semiconductor laser having a front facet reflectance of 60% and rear facet reflectance of 90% is approximately double that of a Fabry-Perot semiconductor laser having a front facet reflectance of 30% and rear facet reflectance of 60% when light outputs are the same.
Furthermore, in a distributed feedback semiconductor laser using an antireflection film for the front facet and a high reflection film for the rear facet, though the light density also depends on the phase of the diffraction grating on the rear facet, there are many device whose light density on the rear facet increases. This tendency becomes noticeable especially when the normalized coupling coefficient is increased in order to reduce a threshold current or when the reflectance of the rear facet is increased, and when assuming that the normalized coupling coefficient is 1.4, the front facet reflectance is 0% and the rear facet reflectance is 90%, the light density on the rear facet is approximately seven times that of the front facet at maximum.
On the other hand, the interface between the semiconductor and the facet coating film is generally a location where there are many interface states and laser deterioration is most likely to occur, and if the facet coating film is designed so that the electric field strength of light at this location becomes a maximum, such an interface is likely to cause deterioration of the laser.
For example, in the case of a resonator of a Fabry-Perot semiconductor laser, a high reflection film is disposed on the rear facet of the resonator as follows. That is, this is a high reflection film in which an SiO2 film of a first layer having a thickness ¼ of in-medium wavelength λ is placed in close contact with the rear facet of the resonator, an amorphous Si (hereinafter described as “a-Si”) film of a second layer having a thickness of λ/4 is superimposed thereon and an SiO2 film of a third layer having a thickness of λ/4 is further superimposed thereon.
In other words, this high reflection film is made up of a low refractive index film having a thickness of λ/4 in close contact with the facet, a high refractive index film having a thickness of λ/4 and a low refractive index film having a thickness of λ/4 on the rear facet of the resonator.
In this case, the electric field strength distribution in the vicinity of the rear facet of the resonator and on the reflection film becomes a maximum at the interface between the rear facet of the resonator and the SiO2 film of the first layer, becomes a minimum at the interface between the SiO2 film of the first layer and the a-Si film of the second layer, becomes a maximum at the interface between the a-Si film of the second layer and the SiO2 film of the third layer and becomes a minimum at the interface between the SiO2 film of the third layer and the air layer.
Since the maximum value at the interface between the a-Si film of the second layer and the SiO2 film of the third layer is smaller than the maximum value at the interface between the rear facet of the resonator and the SiO2 film of the first layer, the electric field strength at the interface formed of different kinds of materials becomes highest at the interface between the rear facet of the resonator and the SiO2 film of the first layer.
For example, the configuration of a high reflection film of a publicly known long-wavelength laser uses a five-layer structure of SiO2/amorphous Si/SiO2/amorphous Si/SiO2 on the facet formed using a cleaving method, and a reflectance of 90% or more is obtained in this way. Alternatively, a λ/4 film made of SiN is formed on the facet, amorphous Si/SiN/amorphous Si/SiN are multilayered on this and a high reflection film having a five-layer structure of SiN/amorphous Si/SiN/amorphous Si/SiN and having a reflectance of 90% is formed (e.g., see Japanese Patent Laid-Open No. 10-290052, paragraphs 0056 and 0057).
The high reflection film in this case is composed of five layers; a low refractive index film having a thickness of λ/4 in close contact with the facet, a high refractive index film having a thickness of λ/4, a low refractive index film having a thickness of λ/4, a high refractive index film having a thickness of λ/4 and a low refractive index film having a thickness of λ/4.
To reduce electric field strength at an interface between facets of a resonator, a semiconductor laser having the following configuration is known.
In this configuration, a dielectric film having a film thickness value of λ/4nc formed of amorphous silicon (refractive index nc to 3.5) having substantially the same refractive index as that of a laser element is provided on the light emission facet of a GaAlAs-based semiconductor laser and a plurality of sets of low refractive index reflection films having a thickness of λ/2nd made up of a dielectric film having a low refractive index (nd) such as an SiO2 film and high refractive index reflection films having a high refractive index are alternately arranged in close contact with the dielectric film of this amorphous silicon. This configuration reduces the electric field strength of light on the light emission facet to a minimum value (e.g., see Japanese Patent Laid-Open No. 63-220589, bottom left field and bottom right field on p. 2).
Furthermore, there is disclosed an example of a semiconductor laser whose oscillating wavelength is approximately 740 nm where an Al2O3 film or SiO2 film having an optical thickness of λ/2 is provided in contact with an facet and multilayered pairs of TiO2 layer and SiO2 layer having an optical thickness of λ/4 are sequentially arranged on the Al2O3 film or SiO2 film of λ/2; three pairs on the light emerging facet side and six pairs on the reflection surface side, which is the opposite side (e.g., Japanese Patent Laid-Open No. 7-45910, paragraphs 0010 and 0011).
Furthermore, when multilayer coating is applied to a semiconductor laser device, a dielectric such as Al2O3, SiO2 or Si3N4 is coated as an odd-numbered layer and Si is coated as an even-numbered layer, but when Si is used for the top layer, Si is oxidized easily, and therefore there is described an example where Si, Al2O3, Si and Al2O3 are sequentially stacked on a cavity facet of a GaAs—GaAlAs-based semiconductor laser having an oscillating wavelength of 8300 Å and an Al2O3 layer is provided on the top layer so as to prevent Si from being oxidized (e.g., Japanese Patent Laid-Open No. 60-130187, from right field on p. 1 to left field on p. 2).
Furthermore, there is disclosed an example where an AlN film is used instead of an Al2O3 film in close contact with an facet of a high output type semiconductor laser element having a large calorific value (e.g., Japanese Utility Model Laid-Open No. 63-162558).
Thus, in order to reduce a threshold current, increasing a reflectance on the facet of a resonator or increasing a normalized coupling coefficient of a diffraction grating causes a light density on the facet of the resonator to increase. Furthermore, in the case of a configuration where electric field strength of light at the interface between the facet of the resonator and reflection film becomes a maximum, when not only the light density on the facet of the resonator is high but also the electric field strength of light becomes a maximum, reliability drastically deteriorates, for example, deterioration of the semiconductor laser is more likely to occur. Furthermore, when mechanical strength at the interface between the facet of the resonator and reflection film is low, heating during assembly of the semiconductor laser device may cause peeling at the interface, leading to a decrease of yield. As such, the configuration of the above described conventional semiconductor laser has a difficulty in simultaneously achieving a reduction of a threshold current and high reliability and may also result in low yield.
In the configuration of the high reflection film of the conventional long-wavelength semiconductor laser described above, when the high reflection film is made up of five layers; a low refractive index film having a thickness of λ/4 in close contact with the facet, high refractive index film having a thickness of λ/4, low refractive index film having a thickness of λ/4, high refractive index film having a thickness of λ/4 and low refractive index film having a thickness of λ/4, as described in Japanese Patent Laid-Open No. 10-290052, there is a problem that the electric field strength of light at the interface between the facet of the resonator and the low refractive index film of the first layer does not always become a minimum value and the electric field strength of light rather increases.
Furthermore, when an a-Si film is formed on the facet of the resonator as described in Japanese Patent Laid-Open No. 63-220589, the adherence of the semiconductor making up the resonator to a-Si Si is not necessarily good. When a laser chip is assembled in a package, heat of soldering is applied thereto, and therefore there is a problem that when the adherence of the a-Si film to the facet of the resonator is insufficient, thermal stress applied thereto may cause the a-Si film on the facet of the resonator to peel off. Furthermore, when a high reflection film is configured, many pairs of low refractive index film and high refractive index film need to be superimposed one upon another, but when the adherence of the a-Si film to the facet of the resonator is insufficient, it may be difficult to superimpose many pairs of low refractive index film and high refractive index film one upon another, resulting in a problem that the yield deteriorates.
Furthermore, as described in Japanese Patent Laid-Open No. 7-45910, when a high reflection film is configured by stacking multilayer pairs of TiO2 layer as the high refractive index film and SiO2 layer as the low refractive index film, in the case of a long-wavelength laser having a wavelength three times or more the wavelength described in Japanese Patent Laid-Open No. 7-45910, it is necessary to stack seven pairs of TiO2 film and SiO2 film to obtain a reflectance of 80% or more in a combination of TiO2 whose refractive index is merely on the order of 2 and SiO2 whose refractive index is 1.40 to 1.45. In this case, there is a problem that the facet coating film becomes extremely thick and peeling of the film is more likely to occur due to thermal stress during assembly.